You don't have to replace the forward lamp harness if you are comfortable with wiring. You only have to add two wires for the amp gauge. GO here for info http://www.azendrud.com/gauges/
This is instructions for the indiot light to gauges conversion but it does cover the wires needed for the amp gauge. Be aware that this site states 70-72 but it will not work for 72 as the bulkhead connector is different for 72.
Here are some instructions for bench testing the gauge cluster so you know it works before going to all the trouble of installing.
You will need at least 4 amps of current.
You will need a 12 v source such as a battery (fuse the pos lead), battery charger or power supply. It is easier if you have a mating connector for the gauge cluster but not necessary. Connect + (12v) to pin 7 (gauge power) or any of the terminals that pin 7 connects to (follow the traces from pin 7) and - (neg) to any clean metal surface of the cluster. To test the dash lights connect another + (12v) wire to pin 2 (Dash lights). To test the oil light (no oil gauge for ss clusters) connect a ground (metal case)to pin 6. To test the turn signal lights connect another +12v to pin 1 for left and pin 12 for right. To test the fuel gauge, connect one end of a 45 ohm resistor to pin 11 and the other end to ground (metal case).
This should registor 1/2. If you don't have any resistors, no connection will be past full and ground will be empty. To test the temp gauge, connect a 75 ohm resistor from pin 10 to ground, this should registor about 210 deg. 50 ohm = 250 deg, 75 ohm = 210 deg and 270 ohm = 100 deg.If you don't have a resistor, open will register far left and ground on pin 10 will register past 250 deg. To test the brake light connect a ground to pin 8. To test the high beam light connect +12v to pin 5. To test the amp gauge connect a AA battery + on pin 3 and - on pin 4, it will registor over 40 amps charge (depending on condition of battery), reverse the AA battery and it will register over 40 amps discharge. To test the tach you will need a pulse generator or a running car that has points or early (no computer) hei ignition. Hook the + from the car battery to pin 7 and - from the car battery to any clean metal surface of the gauge cluster. Hook the negative side of the coil (points ign) or tach terminal(hei ign) to pin 9. start car up and rev motor, tach should respond.
Also, again if you are adept at wiring and you have both the malibu and SS gauge wiring diagrams you can modify the malibu harness to work with the gauges. You will need the 12 pin connector that plugs into the gauge cluster. I have found these in early camaros in the junk yard. Just cut them out of the wire harness leaving about 12in of wire and splice into the malibu harness.